Capacitive micromachined ultrasonic transducers (CMUTs) are proposed as electromechanical transducers that perform at least either of transmission and reception of ultrasonic waves (for example, refer to PCT Japanese Translation Patent Publication No. 2003-527947). The CMUTs are manufactured by using a Micro Electro Mechanical Systems (MEMS) process to which a semiconductor process is applied.
FIG. 7 is a schematic cross-sectional view of a typical CMUT. Referring to FIG. 7, a set of a first electrode 102 and a second electrode 105 is called a cell. The first electrode 102 is opposed to the second electrode 105 with a vibrating membrane 101 and a gap 104 sandwiched therebetween. The vibrating membrane 101 is supported by a supporter 103 formed on a substrate 106. All the first electrodes 102 are electrically connected to each other in the CMUT. A certain level of direct-current (DC) voltage is uniformly applied to the first electrode 102 so that a desired potential difference is generated between the first electrode 102 and the second electrode 105. The second electrodes 105 are electrically separated for every element (a collection of cells). An alternating-current (AC) drive voltage is applied to the second electrode 105 to produce an AC electrostatic attraction between the first and second electrodes, which vibrates the vibrating membrane 101 at a certain frequency to transmit ultrasonic waves. In addition, the vibrating membrane 101 vibrates in response to the ultrasonic waves that are received to generate a minute electric current caused by electrostatic induction at the second electrode 105. The value of the electric current can be measured to acquire a reception signal for every element.
The characteristics in the transmission and reception of ultrasonic waves are determined by the amount of deflection of the vibrating membrane 101 when the DC voltage is applied to the first electrodes 102. The pressure in the gap 104 of each cell is normally lower than the atmospheric pressure, and the vibrating membrane 101 is deflected toward the substrate 106 due to the difference between the atmospheric pressure and the pressure in the gap 104. The amount of deflection of the vibrating membrane 101 is determined by mechanical characteristics based on parameters including the size, shape, thickness, and material of the vibrating membrane 101. When the CMUT is operated, in order to increase the efficiency of the transmission and reception of ultrasonic waves, a certain potential difference is applied between the two electrodes to cause an electrostatic attraction between the electrodes. The vibrating membrane 101 is further deflected toward the substrate 106 due to this electrostatic attraction. In the transmission of ultrasonic waves, the efficiency of the transmission and reception is increased with the decreasing distance between the electrodes because the electrostatic attraction is in inverse proportion to the square of the distance. In contrast, in the reception of ultrasonic waves, the efficiency of the transmission and reception is also increased with the decreasing distance between the electrodes because the magnitude of the detected minute electric current is in inverse proportion to the distance between the electrodes and is in proportion to the potential difference between the electrodes.